The reason you're having problem is the range between the wheels is too big. You should consider a 9" wide wheel for the front or a 9.5" for the rear. Personally I'd go for a 9.5" rear.
What you want to do is stay as close to the circumference of the wheel you're replacing.
Let's say for example, your tires are currently:
Front: 245/45/R17
Rear: 275/40/R17
If you look up those size tires with Kuhmo for the Ecsta MX line, you'll find that they are 25.8" circumference for the fronts and 25.7" for the rears.
So what you're looking for is tires that are near that circumference on the new rim size at similar widths.
If you were to go with an 18x8.5" front and a 18x9.5" rear you could run:
Front: 245/40ZR18
Rear: 275/35ZR18
This would give you a 25.8" tire in the front and a 25.7" tire in the rear. That's the exact same circumference. The bonus is now both tires are seated on a rim at the "measured width" for that tire. Meaning your running the tire on the exact width rim that it was built to run.
If you go with a 8.5" front and a 10" rear, you're going to wind up with a P295 in the rear. That's going to give you a 26.1" tall tire and a 50mm stagger, which is going to make the car trammel a lot.
That's just my $.02, your mileage may vary.
mkiii222;1314134 said:
Or stop being picky and get different brands for front and rear.
That is dangerous advice. Mixing brands is usually not a good idea, you can get radically different handling characteristics out of tires like that, even ones with the same rating.